This invention relates to spark ignition engines and more particularly to an engine that uses intake port injection of ethanol to extend knock limits.
By increasing engine compression ratio and downsizing an engine through turbocharging, spark ignited engines significantly improve their fuel consumption. The fundamental limitation in increasing compression ratios and boost pressures is the phenomenon of engine knock. Knock is a phenomenon in which fuel-air mixture in the end gas auto-ignites before the propagating flame consumes it. Heavy knock can create potentially damaging pressure oscillations. Knock occurs at high fuel-air mixture temperatures and pressures and is affected by fuel chemistry. Direct in-cylinder fuel injection (DI) can be used to alleviate the occurrence of knock. This amelioration of knock results because most of the thermal energy required to evaporate the fuel is extracted from the in-cylinder charge, thus cooling it. In port fuel injected (PFI) engines, only a fraction of the fuel's vaporization heat comes from the air as the fuel is largely sprayed onto the hot intake valve and intake port surfaces. The charge cooling effect is more pronounced for fuels with high heat of vaporization such as ethanol. Additionally, even without the charge cooling effect, ethanol is very effective in preventing knock due to its combustion chemistry. The same effects can be obtained with methanol.
U.S. Pat. No. 7,314,033 of which this application is a continuation-in-part through co-pending application Ser. No. 12/730,662 filed Mar. 24, 2010, discloses an engine concept to avoid knock. One principal embodiment disclosed in the U.S. Pat. No. 7,314,033 uses port fuel injection of gasoline. When needed to avoid knock, this embodiment directly injects an antiknock fuel such as ethanol into the cylinders thereby utilizing both the advantageous chemistry of ethanol and its even more pronounced charge cooling effect to prevent knock. Because the antiknock fuel such as ethanol is used only intermittently, the alcohol direct injector, which is exposed to the high temperature gases within the cylinder, requires a robust design. This is because the alcohol direct injector in that embodiment is not continuously cooled by flowing fuel as is a standard direct injector under most operating conditions.
It is an object of the present invention to provide an engine system that does not require a direct injector that is used only intermittently.